Contaminant particles are among the most common problems associated with IC (integrated circuit) manufacturing processes. They may cause device yield loss because of photo defocus in successive layers, pattern bridging, contact/via opens, CMP (chemical mechanical polishing) scratching, etc. Current in-line inspection tools, such as KLA (an on-line defect inspection tool produced by KLA-Tensor) make it easy to accurately determine the location and size of contaminant particles on wafers but there is no tool for removing these particles except scrubber cleaners. Unfortunately, scrubber cleaning is not a suitable method for dielectric layers, patterned layers, water-absorbent layers, metal layers (corrosion concern), etc and the device yield loss remains the same even though we know the position of these contaminant particles.
Optical traps, sometimes referred to as optical tweezers, utilize a light source to produce radiation pressure. Radiation pressure is a property of light that creates small forces by absorption, reflection, or refraction of light by a dielectric material. Relative to other types of forces, the forces generated by radiation pressure are almost negligible-only a few picoNewtons. However, a force of only a few picoNewtons is sufficient to allow attachment to particles of the sizes just discussed.
Optical tweezers utilize the force that exists when a transparent material with a refractive index greater than the surrounding medium is placed in a light gradient. As light passes through polarizable material, it induces a dipole moment. This dipole interacts with the electromagnetic field gradient, resulting in a force directed towards the brightest region of the light, normally the focal region. Conversely, if an object has a refractive index less than the surrounding medium, such as an air bubble in water, the object experiences a force drawing it toward the darkest region.
As long as the frequency of the laser is below the natural resonances of the particle being trapped, the dipole moment will be in phase with the driving electric field. A schematic view of a light tweezer setup is illustrated in FIG. 1. Light 11, typically laser light, enters a high numerical aperture objective lens 12 of an optical system and is focused 16 to a diffraction limited region (spot) 13 on a particle 14. Because the intensity profile of the laser light is not uniform, an imbalance in the reaction forces generates a three-dimensional gradient force 15 with the brightest light in the center. The gradient force 15 pulls the object toward the brightest point. Thus, the forces generated by the optical system “traps” the object. Such gradient forces are formed near any light focal region.
The sharper or smaller the focal region 13, the steeper the gradient. To overcome scattering forces near the focal region and hence prevent the object from being ejected along the direction of the light beam, the optical system must produce the steepest possible gradient forces. Sufficiently steep gradient forces can be achieved by focusing laser light to a diffraction-limited spot through a microscope objective of high numerical aperture (N.A.).
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,055,106, Grier et al. describe an apparatus for manipulating small dielectric particles. In U.S. Pat. No. 5,953,166, Shikano discloses a laser trapping apparatus while in U.S. Pat. No. 5,689,109 Schutze discloses an apparatus and method for the manipulation, processing and observation of small particles. Weetall et al., in U.S. Pat. No. 5,620,857 use tightly focused laser beams as optical tweezers while Burns et al., in U.S. Pat. No. 5,245,466, create arrays using light beams coupled to microscopic polarizable matter. U.S. Pat. No. 5,079,169 is a method for optically manipulating polymer filaments. Ashkin et al. describe a non-destructive optical trap for biological particles in U.S. Pat. No. 4,893,886. Finally, in U.S. Pat. No. 5,512,745, Finer et al. shows an optical trap system while Ashkin (U.S. Pat. No. 3,808,550), and Shivashankar et al. (U.S. Pat. No. 6,139,831) show optical trap related patents.